Apparatus for processing work pieces, for transmitting and processing information by means of laser emission.
This invention refers to apparatus for processing work pieces, for transmitting and processing information by means of laser emission.
The processing of work pieces by means of laser emission has been known for some years. In particular special laser arrangements, which allow one to attain reproducible processing results (CH-Pat. No. 505,677) at maximum efficiency and optimum processing quality, are known, as for instance as shown in the CH-Pat. Nos. 532,992 and 545,544.
The application of modulated laser pulses for the processing of thermal sensitive working materials, such as brittle, magnetic materials, etc. is shown by CH-Pat. No. 532,993 and CH-application for patent No. 10,089/73.
By the influence of light intensity by means of single regularly following spikes of a duration of approximately 1 .mu.sec, only the material, which is vaporized, is heated. Consequently, the material which surrounds the processing spot remains thermally unaffected. Transparent, as well as reflecting materials can be processed by adapting the peak intensity and the duration of the spikes to the material to be processed.
But for several materials and, specifically, transparent work materials, processing is not possible even with this method or is only possible with extremely high energy single spikes, which results in a small efficiency. This fact is based on insignificant light absorption, which means the coupling, between light wave and lattice vibration, is so small for several materials, as for instance quartz, glass etc. that either nothing is happening, or the light energy heats or influences too large an area. For processing such materials a laser-emission is required which is modulated with a frequency on order of magnitude of the eigen-frequency of one lattice vibration of this material. In other words this means that one has the advantage of the photon-phonon interaction. The absorption of the laser emission is caused by the electric dipole moment of first or second order or by anharmonic terms in the potential energy. Absorption reactions of higher orders may be caused by simultaneous interaction of the radiation field with two or more vibration states of the lattice.
A modulation of the laser emission in the desired frequency range is made possible by so-called "mode-locking". Apart from effects of higher order the mode-locking is a stationary beat of the eigen oscillations of the laser light in the resonator, of such kind, that the product of the pulse length and the bandwidth of the amplifying medium is about one. The spiking is a beat of the intensity of the laserlight with the excited states of the amplifying medium, so that the logarithm of the light intensity approximates a sine function.
The resonance frequencies of mode-locking and spiking lie in totally different frequency ranges:
10 MHz-10 GHz, typically 300 MHz for mode-locking and 10 kHz-10 MHz, typically 300 kHz for spiking.
These two beats can be excited independently and because of the small required depth of modulation both beats can be excited with the same modulator. Under certain circumstances this modulator can in addition be used as output modulator.
Because of different depths of modulations and favourable frequency ranges it is advantageous to adapt the fundamental resonance of the modulator to the spiking frequency.
Simultaneous Q-switching and mode-locking with a single modulator inside of a resonator is known from the U.S. Pat. No. 3,763,443. This is obtained by modulation with a FM-signal, the center frequency of which is out-of-tune with respect to the mode-locking frequency. However the depth of modulation has to be almost one, what complicates a practical application.
A method for amplitude and phase coupling of several equidistant laser transitions in molecular lasers, for instance CO.sub.2 lasers, is known from the U.S. Pat. Nos. 3,550,031 and 3,493,894. This method however produces only a regular train of single pulses with a separation given by the laser gas.
It is known, that the biggest bandwidth for information transmittance is obtained, when pulse trains are modulated parallely in many channels and by using the time multiplex method. Therefore a high frequency modulator of narrow band and a modulator of broadband are used, as is shown in CH-Pat. No. 541,816. For data processing high frequency modulated light sources of narrow band are needed, as for instance in optical transformations and correlators in holography and for instance in demultiplexing of the above mentionned information trains (CH-Pat. No. 553,459).
Corresponding to the state of technique more than one modulator is required in general for generation of a modulation and of mode-locking and, if needed, for control of the pulse train. But this is very disadvantageous and it may be expensive too. Additionally the conventional modulation systems have low driving oscillation efficiency.
The purpose of this invention is to create a method of and an apparatus for the processing of work pieces and for transmitting and processing information by laser emissions; a method and an apparatus which generate simultaneously mode-locking and spiking with a single modulator and which further make possible control of pulsetrains. Additionally the energy transfer has to be increased.
The method according to the invention is characterized by a temporal emission of the laser in form of regular spikes which can be resolved in single very short pulses by mode-locking. This approach is characterized by the possibility of a control of the intensity of the spike train.
By this method the transfer of light energy in the material to be processed is optimum, because the mode-locking excites acoustical phonons caused mainly by electrostriction. Therefore work material, which is extremely resistant to generally used laser emission can be processed by this method.
Additionally the apparatus to perform the given task has to enable a modulation of a narrow band.
The apparatus according to the invention is characterized by adding a modulator to the laser resonator generating as well a phase modulation as an amplitude modulation of the laser radiation and therefore being able to produce simultaneously a modulation (spiking), a mode-locking and a control of the intensity of the spike train.
This apparatus allows the generation of the spiking and the mode-locking by extremly small driving voltage (.about.1 V). The laser resonance for mode-locking is sharp: Q-value .gtoreq.10.sup.3, and for the spiking flat: Q-value.about.3. By using the modulator as out-put modulator a third signal outside of the two resonances can control simultaneously the degree of the laser out-put.
For this out-put control one has to take care, that the mode-locking and spiking is not disturbed by the shift of the working point caused by the needed large modulation depth.
In all applications it is reasonable to store the unused power in the laser resonator, especially at high powers. Therefore, out-put modulators enable optimum energy profit.
Outside the resonances the needed driving power for a given modulation depth and frequency component is proportional to the modulation frequency. In the resonance the needed driving power is indirectly proportional to the mechanical resonator quality Q.sub.m. In addition, the attainable relative band-width is also indirectly proportional to the mechanical resonator quality. The minimum driving power per band-width can be reached by adapting the mechanical resonator quality Q.sub.m to the band-width .DELTA.f and by adapting the eigen-frequency of the modulator to the carrier frequency f. EQU fo = f EQU .DELTA.f .sup.. Q.sub.m .apprxeq. fo
The maximum attainable mechanical resonator quality is limited by the losses of the mechanical and partially electrical oscillators in the resonator and it can reach values on the order of magnitude of 10.sup.6 in quartz and 10.sup.8 in Li Nb O.sub.3.